1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a liquid crystal display (LCD) device having patterned spacers.
2. Discussion of the Related Art
Recently, liquid crystal display (LCD) devices have been widely used for notebook computers and desktop monitors, etc. because of their superior resolution, color image display and quality of displayed images. In general, an LCD device has an upper substrate and a lower substrate, which are spaced apart and facing each other, and a liquid crystal layer disposed between the upper and lower substrates. Each of the substrates includes an electrode, and the electrodes of each substrate are also facing each other. The LCD device uses an optical anisotropy of liquid crystal and produces an image by controlling light transmissivity by varying the arrangement of liquid crystal molecules, which are arranged by an electric field.
Because LCD devices have high resolution and can display an excellent moving image, they are widely used. The LCD device typically includes thin film transistors and pixel electrodes arranged in a matrix form. The LCD device is referred to as an active matrix liquid crystal display (AMLCD).
The LCD device is fabricated by forming a lower substrate, referred to as an array substrate, having thin film transistors (TFTs) and pixel electrodes; forming an upper substrate, referred to as a color filter substrate, having common electrodes and color filters; forming a liquid crystal cell by aligning and attaching the substrates; injecting liquid crystal materials and sealing; and attaching a polarization film.
FIG. 1 is a cross-sectional view illustrating a conventional liquid crystal display (LCD) device. In FIG. 1, the conventional LCD device has upper and lower substrates 10 and 30, which are spaced apart and facing each other, and also has a liquid crystal layer 50 interposed between the upper and lower substrates 10 and 30.
A gate electrode 32 is formed on the inside of the lower substrate 30, and a gate insulating layer 34 covers the gate electrode 32. An active layer 36 is formed on the gate insulating layer 34 over the gate electrode 32, and an ohmic contact layer 37 is formed on the active layer 36. Next, source and drain electrodes 38 and 40 spaced apart from each other are formed on the ohmic contact layer 37. The gate electrode 32, the active layer 36, the source electrode 38, and the drain electrode 40 constitute a thin film transistor T. The active layer 36 between the source and drain electrodes 38 and 40 becomes a channel CH of the thin film transistor T.
Although not shown in the figure, a gate line, which is connected to the gate electrode 32, is formed in a first direction and a data line, which is connected to the source electrode 38, is formed in a second direction. The gate line and the data line cross each other, and define a pixel region P.
Next, a passivation layer 42 is formed on the thin film transistor T. The passivation layer 42 has a drain contact hole 44 exposing the drain electrode 40. A pixel electrode 48 is formed in the pixel region P on the passivation layer 42. The pixel electrode 48 is connected to the drain electrode 40 through the drain contact hole 44.
A black matrix 12, which has an opening corresponding to the pixel electrode 48, is formed on the inside of the upper substrate 10. A color filter layer 14, which transmits only light having a certain color corresponding to the opening of the black matrix 12, is formed on the black matrix 12. The color filter layer 14 includes three sub-color filters of red (R), green (G) and blue (B). Each color filter corresponds to respective pixel electrodes 48. The black matrix 12 covers the thin film transistor T and thus prevents light from going into the channel CH of the thin film transistor T. In addition, the black matrix 12 blocks light leakage in a border portion between adjacent sub-color filters 14. Subsequently, a transparent common electrode 16 is formed on the color filter 14 as an electrode for applying voltage to the liquid crystal layer 50.
A seal pattern 52 is formed in a peripheral portion between the upper and lower substrates 10 and 30. The seal pattern 52 prevents liquid crystal materials of the liquid crystal layer 50 from leaking.
Meanwhile, a ball spacer 54 is formed in the pixel region P between the upper and lower substrates 10 and 30 to maintain uniform cell gap with the seal pattern 52.
Although not shown in the figure, alignment layers are formed on the pixel electrode 48 and the common electrode 16, respectively, to arrange liquid crystal molecules of the liquid crystal layer 50.
The ball spacer 54 may be made of glass wool or organic material having elasticity to outer pressure. By the way, ball spacers may cause the following problems because the ball spacers are randomly scattered on a substrate.
First, the alignment layers become bad due to movements of the ball spacers. Second, light leakage may occur around the ball spacers by absorptive forces between the liquid crystal molecules adjacent to the ball spacers. Third, when the ball spacers are used in a large sized liquid crystal display device, it is hard to maintain stable cell gap. Fourth, since the ball spacers have elasticity and are not fixed, ripple phenomena may happen when a screen is touched. As a result, in a liquid crystal display device, which maintains the cell gap by using the ball spacers, high quality images are difficult to achieve.
To solve the problems, patterned spacers, which may be formed through a photolithography process, have been proposed and developed. The patterned spacers enable-uniform cell gap to be maintained. Because the patterned spacers can be fixedly formed in non-pixel area, the patterned spacers can block light leakage around themselves and the device can be solid. Additionally, in the case of requiring narrow cell gap, the cell gap of the device can be minutely controlled by the patterned spacers. Furthermore, when the screen is touched, a ripple phenomena can be prevented.
FIG. 2 is a schematic cross-sectional view of a liquid crystal display device having a patterned spacer according to the related art.
As shown in FIG. 2, an upper substrate 60 and a lower substrate 70 are spaced apart from and facing each other. A thin film transistor T and a pixel electrode 72 are formed on an inner surface of the lower substrate 70. The pixel electrode 72 is connected to the thin film transistor T and is made of a transparent conductive material. A black matrix 62 is formed on an inner surface of the upper substrate 60 to cover the thin film transistor T, and a color filter layer 64 is formed on the black matrix 62. A common electrode 66 made of the same material as the pixel electrode 72 is formed on the color filter layer 64.
To maintain a uniform cell gap between the upper and lower substrates 60 and 70, a pattern spacer 74 is formed to correspond to the black matrix 62 and the thin film transistor T.
A liquid crystal layer 80 is interposed between the upper and lower substrates 60 and 70. Although not shown in the figure, upper and lower alignment layers are formed on the pixel electrode 72 and the common electrode 66, respectively.
In the related art, the patterned spacer is formed on either the upper substrate 60 or the lower substrate 70, and then the upper and lower substrates 60 and 70 are attached by using the patterned spacer, whereby a certain cell gap is formed between the upper and lower substrates 60 and 70.
The thickness of the patterned spacer grows thick in proportion to the cell gap, and the accuracy of a pattern is decreased as the patterned spacer becomes thick. Therefore, a uniform cell gap is hard to achieve due to poor planarization characteristics, and badly rubbed portions are increased.
FIG. 3 is a schematic cross-sectional view of showing a process rubbing a substrate for a liquid crystal display device according to the related art, and the substrate includes an alignment layer.
As shown in FIG. 3, a patterned spacer 84 is formed on a substrate 82, and an alignment layer 86 is formed on an entire surface of the substrate 82 to cover the patterned spacer 84.
A rubbing process is performed to form grooves on the surface of the alignment layer 86 in a certain direction by using a rubbing fabric 88. At this time, a portion C adjacent to a side of the patterned spacer 84, which is disposed behind the patterned spacer 84 with respect to a rubbing direction, is not rubbed or is irregularly rubbed. Moreover, the portion C grows wider as a thickness H of the patterned spacer increases. For example, if the patterned spacer has a thickness of about 5 μm, the poorly rubbed portion around the patterned spacer may have a size within a range of about 7 μm to about 8 μm.
Since the poorly rubbed portion should be covered with the black matrix, the aperture ratio of the device is lowered due to an increasing size of the black matrix. On the other hand, the patterned spacer is not suitable in a device having a cell gap of more than 5 μm.